1. Field of the Invention
The present invention relates to an optical device of a small size. More particularly, the present invention relates to an optical device miniaturized by allowing light to operate in a confined small space. Such optical devices are particularly suitable for optical pick-up devices, laser printers, and sensors, which require little space and light weight. In general, the optical device has two types; an integrated type and a substrate type.
2. Description of the Prior Art
In recent years, various kinds of optical information processors have been developed. Typical examples are compact disks that are widely used instead of gramophone records, electronic filing systems for storing documents and photographs in magneto-optical disks, and laser printers for writing information in a recording section of a copying machine with a laser beam. An optical information processor is also used in automated manufacturing equipment as an optical encoder and an optical range finder.
For example, a compact disk system is provided with pits having a depth of about 1/2 wavelength which are intended to indicate digital signals "1" and "0". By reading data in terms of the pits the data is reproduced on a phonograph record.
The pit data is optically read so that large spaces are not required for one bit data but only a tiny area of about 1 .mu.m.sup.2 is sufficient. This leads to a highly dense recording system. Because of this advantage, compact disks are becoming more popular than magnetic tapes or magnetic disks.
The compact disk requires an optical component such as a pick-up device for reading pit data. The optical components include a lens, a beam splitter and other components, and the assemblage and adjustment of these are time- and labor-consuming. In addition, the size is likely to become large and heavy, and the cost is likely to be high.
In order to explain the background in more detail, reference will be made to FIG. 5 which shows a known optical pick-up device intended to overcome the problems mentioned above. This optical device is a self-contained system 59 which includes a hologram 55 formed on a glass plate 52 so as to enable it to function as a beam splitter and a diffracting grating, a semiconductor laser 51 as an optical source and an optical detector 56 for detecting signals. The pick-up device is operated as follows:
A laser beam D emitted upward from the semiconductor laser 51 is focused on an optical disk 57 through a collimator lens 53 and a focusing lens 54 both disposed above the hologram 55. The light reflecting from the disk 57 reversely follows the above-mentioned path but is deflected by the hologram 55, the light being detected by the detector 56. The focusing lens 54 is adjusted as to its position and focal point by means of an actuator (not shown) using an electro-magnet.
This known pick-up device is comparatively lightweight because of a reduced number of components as well as a reduced size of each component, but ideally, the desired weight should be much lighter. Among the components, it is only the focusing lens 54 that can be moved by the actuator because of its light weight.
FIG. 6 shows another pick-up device designed to solve the problems pointed out above. A typical example of this device is disclosed in Japanese Laid-Open Patent Publication No. 62-117150. A three beam generating grating 62, a collimator diffracting grating 63, astigmatism generating lenses 76 and 77, a semiconductor laser 71 and an optical detector 78 are all mounted on a glass plate, except for a focusing lens 75 for focusing a source light on the disk 57.
This arrangement can dispense with any optical adjustment between one component and other. There is little possibility of causing displacement or deflection due to mechanical vibration or shock. This type of pick-up device will be referred to as "substrate type pick-up".
A substrate type pick-up device can be smaller in size than an ordinary pick-up device because the source light is propagated through the substrate 61 even though the distance between the light source and optical disk 57 is the same.
However, the small pick-up device requires the laser beam to be as small as possible in diameter so as to pass through the focusing lens without divergence. As a result, the advantage of the small size is traded off against the necessity for requiring a highly precision focusing lens 75. The focusing lens 75 is not mounted on the substrate 61 which prevents the size of the pick-up device from being reduced beyond a certain limit.
In order to make most use of the characteristics of light, the beam must be sufficiently focused on the optical disk 57. The minimum radius W.sub.0 of the beam focused by an ordinary lens, the numerical aperture NA of the lens and the wavelength .lambda. of the beam are in the following relationship: EQU W.sub.0 =.lambda./NA (1)
The numerical aperture NA is nearly equal to d/2f where d is the smaller of the diameter of the lens or the diameter of the laser beam (i.e. the effective diameter of the lens), and f is the focal length of the lens. The minimum diameter 2W.sub.0 of the beam is called "diffraction limit".
If a lens has a larger numerical aperture NA, the beam can be focused smaller but this results in more aberration and difficulty in fabrication. If the device is to be made compact, the diameter of the beam must be reduced, but in order to maintain the numerical aperture NA constant, the focal length must be reduced. This requires the lens face to have a small curvature, but it is likely to cause aberration. Since a substrate type pick-up device has a relatively short distance from the light source to the focusing lens 75, the effective diameter of the focusing lens 75 can be relatively small. As a result, it is difficult to produce focusing lenses having a large numerical aperture NA.
Any other optical systems such as hologram laser printers cannot be miniaturized without sacrificing the focusability of the lens. The reduction in the focusability restricts the range of application.
In order to focus a light beam to below the minimum diameter decided by the numerical aperture NA of the lens, another method is proposed which consists of blocking light from passing through a central portion of the focusing lens 75. According to this method, the light beams passing through the peripheral portion of the lens are focused, thereby intensifying the secondary peaks appearing on both sides of the primary peak on the disk 57. The primary peak becomes narrower than when the light passing through the central portion is not blocked. This means that the light beam is focused beyond the minimum diameter of the beam determined by the numerical aperture NA of the lens. This phenomenon is called "hyperresolution effect", and such lenses are called "hyperresolution lenses".
Under a known system a hyper-resolution lens is disposed separately from the optical pick-up system. This necessitates the movement of the lens in accordance with that of the pits in the optical disk 57. Because of this movement, the light beam is likely to deviate from the central portion of the lens, thereby failing to achieve the desired focusing characteristics.
As is evident from the foregoing description, a light beam cannot be focused beyond a certain limit, thereby making it difficult to achieve a miniaturized optical pick-up device.